Many modern vehicles employ four wheel drive systems. These systems have been marketed in two forms. Systems generally termed four wheel drive (4WD) have a transfer case which is controlled by the operator to select two wheel or four wheel drive. If the operator selects the four wheel drive condition, the vehicle drives all four wheel continuously. Some of these systems have employed overrunning clutches at two of the wheel to alleviate some of the disadvantages of 4WD which result from tire pressure differential and cornering to name a few.
All wheel drive (AWD) systems also provide the benefits of a four wheel drive vehicle and do not require the operator to intentionally select this condition. These systems often employ a viscous clutch in the center differential to transfer torque to the drive wheels that are not sensed as slipping. In tight cornering situations and during towing, these AWD systems present a disadvantage. The vehicle must be placed on a flat-bed type towing vehicle to prevent overheating of the powertrain during towing. In cornering situations, noise and vibration can result from the AWD system being engaged. While this is not detrimental to the powertrain during short durations, it can be disconcerting to the operator.